The exploration of gas and oil involves drilling a borehole into the ground through the geological formations of interest. Drilling mud is used to lubricate and cool the drill bit, to assist in bringing the drill cuttings to the surface, to provide sufficient hydrostatic pressure within the well to prevent the bore collapsing under the forces experienced underground, and to prevent the influx of fluid from the formation while drilling. The drilling mud can be either aqueous based or oil or synthetic based.
When the drilling of the borehole is completed a tubular known as a casing is secured within the borehole with cement. This insures permanence of the borehole and it prevents entry of formation fluid, other than from the formation which is being produced. It is common procedure to pump the cement down the inside of the casing to the bottom and then up the outside of the casing, between the casing and the borehole wall, until it fills and lines the section of the annulus to be cemented. That is, the space between the casing and the borehole wall. It is important that the cement lining be free of voids, that is, sections between the casing and borehole wall that is void of cement. It is also important that the cement be well bonded to both the casing and borehole wall. If good bonding is not established between the cement and the casing and borehole wall surfaces, then production from one reservoir could enter another and be lost; an undesirable fluid from one reservoir could enter the production zone of another; or the pumped fluid of an injection well could end up in the wrong zone. To ensure good bonding between cement and the appropriate surfaces, it is necessary to remove substantially all traces of drilling mud on the casing and the borehole wall. Incomplete mud removal can leave a channel behind which could prevent total isolation of a production zone. Unfortunately, the substantial complete removal of mud often proves extremely difficult. Various methods and mechanisms have been devised in an attempt to achieve complete mud removal and to provide a complete cement lining, free of voids, between the casing and the borehole wall.
One type of method includes the use of spacer fluids ahead of the cement. That is, liquids that will literally wash the mud off of the casing and borehole wall. These fluids can be pumped so that they are positioned between the cement and the mud. Such fluids can be of the type known as "chemical washes" which are usually low-viscosity liquids containing surfactants and mud thinners, or diluents. They can also be those known as "spacers", which are rather more viscous, gel-like liquids that are primarily used to form a buffer between the cement and the mud. Although it is not always clear in the literature whether a particular fluid is a spacer or a chemical wash, a spacer is generally characterized as a thickened composition that functions primarily as a fluid piston in displacing the mud. Frequently, spacers contain an appreciable amount of weighting materials, as well as fluid loss control agents. Chemical washes, on the other hand, are generally thin, or relatively non-viscous, fluids which are effective principally as a result of turbulence, dilution, and surfactant action on the mud and mud filter cake. Chemical washes may contain some solids to act as an abrasive, but the solids content is generally significantly lower than in spacers because chemical washes are typically too low in viscosity to have good solids carrying capacity. For purposes of the present invention, the term "spacer" or "spacer fluid" means both chemical washes as well as the more conventional meaning for the more viscous spacer fluids.
Aqueous based chemical washes are taught in U.S. Pat. No. 4,207,194. These chemical washes are of the type containing one or more of: (a) at least one surfactant to remove water based drilling muds from a borehole; and (b) at least one surfactant to enhance the bonding of cement to the wall of a borehole and the casing. There is also included an effective amount of a fluid loss additive such as a mixture of at least two oil soluble particulate resins, one of which remains hard and friable, and the other of which is soft and pliable (at the temperature to be encountered in the well) when dispersed in an aqueous medium.
U.S. Pat. No. 4,108,779 teaches a water-in-oil emulsion spacer fluid which is compatible with drilling fluids and cement compositions and which is comprised of a hydrocarbon oil, salt water, an emulsifier which is comprised of a first and second fatty acid, and a surfactant-dispersant.
U.S. Pat. No. 5,101,902 teaches an aqueous spacer composition compatible with drilling muds and cement slurries which spacer composition is comprised of a biopolymer selected from the group consisting of scleroglucan and biopolymers produced by the bacteria Azotobacher indicus as an anti-settling means, a fluid loss controlling means, a wetting agent selected from the group consisting of at least one polyethoxyl ester of a C.sub.9 -C.sub.12 fatty acid, and a weighting means.
U.S. Pat. No. 4,588,032 teaches a fluid spacer composition for use in well cementing, which composition is comprised of an aqueous fluid, an admixture of nonylphenols ethoxylated with from 1 to 14 moles of ethylene oxide, and preferably, at least one member selected from the group consisting of a sulfonated linear C.sub.8 to C.sub.18 straight chain alcohol ethoxylated with from 2 to 20 moles of ethylene oxide, a low molecular weight aliphatic glycol ether containing from about 3 to about 12 carbon atoms per molecule and an alcohol containing from 3 to 8 carbon atoms ethoxylated with 2 to 4 moles of ethylene oxide together with any conventionally utilized spacer additives, if desired, such as viscosifiers, weighting agents, and the like.
While there are various chemical washes and spacer fluids in commercial practice today, there is still a need in the art for ever improved techniques for displacing drilling fluids and leaving the borehole surfaces clean for adequate bonding of cement in the annular space defined by the casing and borehole wall.